Germany's biggest airport was scrambling to book passengers on new flights on Thursday after a massive storm caused dozens of cancellations, and also triggered flooding that deluged streets and toppled trees.

The storm swept over southwest Germany late Wednesday, dumping huge quantities of water and reportedly unleashing over 25,000 bolts of lightning in about an hour.

Frankfurt Airport said it was forced to axe 90 flights while 23 more were rerouted to land at other airports.

Some passengers slept on camp beds while many spent the night at the airport hotel.

"Today, the situation will probably stabilise again," a spokesman told AFP.

"There are still passengers here who need to be booked onto new flights." 

The fire service in Germany's financial capital said they launched over 500 operations from late Wednesday to early Thursday related to the storm. 

There were more than 350 instances of water flooding into buildings and 17 fallen trees, they said, adding off-duty personnel were called in to help.

The storm also affected other areas, with Gelsenkirchen, in the region North Rhine-Westphalia, hard hit.

Streets, cellars and lower-lying residential areas were rapidly flooded, while trees toppled over, hitting vehicles, the fire service said.

Emergency service workers rescued people from vehicles at several highway underpasses. 

In one area, parked vehicles became submerged and some streets were only passable with inflatable boats. 

Experts say that climate change is increasing the frequency and severity of floods. 

In 2021, Germany's western regions of Rhineland-Palatinate and North Rhine-Westphalia were hit by catastrophic floods that killed over 180 people.